.\" $OpenBSD: EVP_PKEY_size.3,v 1.3 2024/03/05 19:21:31 tb Exp $
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.Dd $Mdocdate: March 5 2024 $
.Dt EVP_PKEY_SIZE 3
.Os
.Sh NAME
.Nm EVP_PKEY_size ,
.Nm EVP_PKEY_bits ,
.Nm EVP_PKEY_security_bits
.Nd EVP_PKEY information functions
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_PKEY_size
.Fa "const EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_PKEY_bits
.Fa "const EVP_PKEY *pkey"
.Fc
.Ft int
.Fo EVP_PKEY_security_bits
.Fa "const EVP_PKEY *pkey"
.Fc
.Sh DESCRIPTION
.Fn EVP_PKEY_size
returns the maximum size in bytes needed for the output buffer
for almost any operation that can be done with
.Fa pkey .
The primary use is with
.Xr EVP_SignFinal 3
and
.Xr EVP_SealInit 3 .
The returned size is also large enough for the output buffer of
.Xr EVP_PKEY_sign 3 ,
.Xr EVP_PKEY_encrypt 3 ,
.Xr EVP_PKEY_decrypt 3 ,
and
.Xr EVP_PKEY_derive 3 .
.Pp
Unless the documentation for the operation says otherwise,
the size returned by
.Fn EVP_PKEY_size
is only an upper limit and the final content of the target
buffer may be smaller.
It is therefore crucial to take note of the size given back by the
function that performs the operation.
For example,
.Xr EVP_PKEY_sign 3
returns that length in the
.Pf * Fa siglen
argument.
.Pp
Using
.Fn EVP_PKEY_size
is discouraged with
.Xr EVP_DigestSignFinal 3 .
.Pp
Most functions using an output buffer support passing
.Dv NULL
for the buffer and a pointer to an integer
to get the exact size that this function call delivers
in the context that it is called in.
This allows those functions to be called twice, once to find out the
exact buffer size, then allocate the buffer in between, and call that
function again to actually output the data.
For those functions, it isn't strictly necessary to call
.Fn EVP_PKEY_size
to find out the buffer size, but it may still be useful in cases
where it's desirable to know the upper limit in advance.
.Pp
By default,
.Fn EVP_PKEY_size
is supported for the following algorithms:
.Bl -column ED25519 "EVP_MAX_BLOCK_LENGTH = 32"
.It        Ta same result as from:
.It CMAC   Ta Dv EVP_MAX_BLOCK_LENGTH No = 32
.It DH     Ta Xr DH_size 3
.It DSA    Ta Xr DSA_size 3
.It EC     Ta Xr ECDSA_size 3
.It ED25519 Ta 64, but see below
.It HMAC   Ta Dv EVP_MAX_MD_SIZE No = 64
.It RSA    Ta Xr RSA_size 3
.It X25519 Ta Dv X25519_KEYLEN No = 32
.El
.Pp
For
.Dv EVP_PKEY_ED25519 ,
the situation is special: while the key size is
.Dv ED25519_KEYLEN No = 32 bytes ,
.Fn EVP_PKEY_size
returns 64 because the signature is longer than the keys.
.Pp
The application program can support additional algorithms by calling
.Xr EVP_PKEY_asn1_set_public 3 .
.Pp
.Fn EVP_PKEY_bits
returns the cryptographic length of the cryptosystem to which the key in
.Fa pkey
belongs, in bits.
The definition of cryptographic length is specific to the key cryptosystem.
By default, the following algorithms are supported:
.Bl -column ED25519 "the public domain parameter p" DSA_bits(3)
.It        Ta cryptographic length = Ta same result as from:
.It        Ta significant bits in ... Ta
.It DH     Ta the public domain parameter Fa p Ta Xr DH_bits 3
.It DSA    Ta the public domain parameter Fa p Ta Xr DSA_bits 3
.It EC     Ta the order of the group Ta Xr EC_GROUP_order_bits 3
.It ED25519 Ta 253 Ta \(em
.It RSA    Ta the public modulus Ta Xr RSA_bits 3
.It X25519 Ta 253 Ta \(em
.El
.Pp
The application program can support additional algorithms by calling
.Xr EVP_PKEY_asn1_set_public 3 .
.Pp
.Fn EVP_PKEY_security_bits
returns the security strength measured in bits of the given
.Fa pkey
as defined in NIST SP800-57.
By default, the following algorithms are supported:
.Bl -column ED25519 DSA_security_bits(3)
.It        Ta same result as from:
.It DH     Ta Xr DH_security_bits 3
.It DSA    Ta Xr DSA_security_bits 3
.It EC     Ta Xr EC_GROUP_order_bits 3 divided by 2
.It ED25519 Ta 128
.It RSA    Ta Xr RSA_security_bits 3
.It X25519 Ta 128
.El
.Pp
For EC keys, if the result is greater than 80, it is rounded down
to 256, 192, 128, 112, or 80.
.Pp
The application program can support additional algorithms by calling
.Xr EVP_PKEY_asn1_set_security_bits 3 .
.Sh RETURN VALUES
.Fn EVP_PKEY_size
and
.Fn EVP_PKEY_bits
return a positive number or 0 if this size isn't available.
.Pp
.Fn EVP_PKEY_security_bits
returns a number in the range from 0 to 256 inclusive
or \-2 if this function is unsupported for the algorithm used by
.Fa pkey .
It returns 0 if
.Fa pkey
is
.Dv NULL .
.Sh SEE ALSO
.Xr EVP_PKEY_decrypt 3 ,
.Xr EVP_PKEY_derive 3 ,
.Xr EVP_PKEY_encrypt 3 ,
.Xr EVP_PKEY_new 3 ,
.Xr EVP_PKEY_sign 3 ,
.Xr EVP_SealInit 3 ,
.Xr EVP_SignFinal 3
.Sh HISTORY
.Fn EVP_PKEY_size
first appeared in SSLeay 0.6.0 and
.Fn EVP_PKEY_bits
in SSLeay 0.9.0.
Both functions have been available since
.Ox 2.4 .
.Pp
.Fn EVP_PKEY_security_bits
first appeared in OpenSSL 1.1.0 and has been available since
.Ox 7.2 .
